1. Field of the Invention
The present invention relates to a driving transmission device which transmits torque of a driving source to a driven unit by a belt wound on a driving pulley and a driven pulley, and an image forming apparatus.
2. Description of the Related Art
In general, various apparatuses including image forming apparatuses, such as copiers and printers, have a driving transmission device which transmits torque of a driving source, such as a motor, to a driven unit.
For example, a configuration in which a driving force of a motor serving as a driving source is transmitted to a driving roller which drives a photoconductor drum and an intermediate transfer belt which are driven unit through gears is employed in image forming apparatuses. However, in this configuration, a difference between rotation transmission of a driving gear and rotation transmission of a driven gear (for example, a mesh transmission difference) causes vibration. The generated vibration may be transmitted to gear supporting members, such as a shaft, bearings, and side plates, and driven members, and accordingly, large noise may be generated. If vibration is generated in such an image forming apparatus, a stripe image or an image having uneven density may be generated or deterioration, such as streaks or unevenness, may be generated in an image.
Accordingly, a driving transmission device having a configuration in which a belt is wound on a pulley connected to a driving source and a pulley connected to a driven unit has been developed. The driving transmission device has the pulleys and the belt absorbed by each other due to electrostatic absorption, and transmits the driving force of the driving source to the driven unit. An image forming apparatus disclosed in Japanese Patent Laid-Open No. 8-146783 has a configuration in which a driving roller drives an intermediate transfer belt. The image forming apparatus applies a bias voltage to a core metal of the driving roller so that an electrostatic absorption force is generated between the driving roller and the intermediate transfer belt and a driving force of the driving roller is transmitted to the intermediate transfer belt.
When a voltage is applied between a conductive portion of the belt and conductive portions of the pulleys so that an electrostatic force (Johnsen-Rahbek force) is generated, current is supplied to dielectric layers interposed between the belt and the pulleys and Joule heat is generated.
Here, a length of a portion of a belt which is wound on a pulley (winding length) and an electric resistance value are inversely proportional to each other. As an area of a portion of a pulley which is in contact with a belt is increased, a resistance value between the pulley and the belt is reduced. For example, in a case where a portion of a pulley on which a belt is wound is large in length, an area of a portion of the pulley which is in contact with the belt through a dielectric layer is large when compared with a case where a portion of the pulley on which the belt is wound is small in length. Therefore, a total value of resistance between a conductive portion of a pulley having a long winding length and a conductive portion of a belt is smaller.
Accordingly, if the same voltage is applied between a driving pulley and a belt and between a driven pulley and the belt so that the pulleys and the belt are absorbed with each other by an electrostatic force, an amount of current of the pulley having a small resistance value, that is, the pulley having a long winding length, is increased. Therefore, Joule heat is excessively increased in the pulley having a long winding length, and consequently, amounts of heat generation of the pulley and the belt are increased.